Eric Eslinger

Mechanism of burial metamorphism of argillaceous sediment: 1. Mineralogical and chemical evidence

A detailed mineralogical and chemical investigation has been made of shale cuttings from a well (Case Western Reserve University Gulf Coast 6) in Oligocene-Miocene sediment of the Gulf Coast of the United States. The 10-µm fractions from the 1,250- to 5,500-m stratigraphic interval were analyzed by x-ray diffraction. Major mineralogic changes with depth take place over the interval 2,000 to 3,700 m, after which no significant changes are detectable. The most abundant mineral, illite/smectite, undergoes a conversion from less than 20 percent to about 80 percent illite layers over this interval, after which the proportion of illite layers remains constant. Over the same interval, calcite decreases from about 20 percent of the rock to almost zero, disappearing from progressively larger size fractions with increasing depth; potassium feldspar (but not albite) decreases to zero; and chlorite appears to increase in amount. Variations in the bulk chemical composition of the shale with depth show only minor changes, except for a marked decrease in CaO concomitant with the decrease in calcite. By contrast, the <0.1-µm fraction (virtually pure illite/smectite) shows a large increase in K2O and Al2O3 and a decrease in SiO2 The atomic proportions closely approximate the reaction smectite + Al+3 + K+ = illite + Si+4. The potassium and aluminum appear to be derived from the decomposition of potassium feldspar (and mica?), and the excess silicon probably forms quartz. We interpret all the major mineralogical and chemical changes as the response of the shale to burial metamorphism and conclude that the shale acted as a closed system for all components except H2O, CaO, Na2O, and CO2. Compositional changes in the shale as a function of metamorphic grade closely parallel compositional changes in shale as a function of geologic age.

Chemistry of illite/smectite and end-member illite

Chemical data from three different series of diagenetic illite/smectites (I/S), analyzed statistically by two regresion techniques, indicate that the content of fixed-K per illite layer is not constant, but ranges from ~0.55 per O10(OH)2 for illite layers in randomly interstratified I/S (R=0; >50% smectite layers) to ~ 1.0 per O10(OH)2 for illite layers formed in ordered I/S (R>0; <50% smectite layers). By extrapolation of the experimental data, the following chemical characteristics were obtained for end-member illite derived from the alteration of smectite in bentonite: average fixed-K per illite layer = 0.75 per O10(OH)2; total charge = about -0.8; cation-exchange capacity = 15 meq/100 g; surface area (EGME) = 150 m2/g.

ROLE OF IRON REDUCTION IN THE CONVERSION OF SMECTITE TO ILLITE IN BENTONITES IN THE DISTURBED BELT, MONTANA

Cretaceous bentonites were collected in outcrop from the Sweetgrass Arch and the Disturbed Belt in Montana. The mixed-layer illite-smectite (I/S) components of the bentonites from the Sweetgrass Arch have from 0 to 25% illite layers and no detectable structurai Fe2+, whereas the samples from the Disturbed Belt have from about 25 to 90% illite layers, and all contain Fe2+. A positive correlation (r = 0.89) exists between the percentage of structural iron that is Fe2+ and the amount of fixed interlayer K in the I/S.The higher percentage of illite layers in the samples from the Disturbed Belt is attributed to reactions related to elevated temperatures caused by burial beneath thrust sheets. The increase in Fe2+/Fe3+ with increasing percentages of illite layers is tentatively attributed to a redox reaction involving the oxidation of organic matter. Although there is no statistical evidence for an increase in octahedral charge with an increase in illite layers when all the samples are considered together, iron reduction may have contributed as much as 10 to 30% of the increase in total structural charge that occurred in any given sample during metamorphism. The remaining structural charge increase can be attributed to the substitution of Al3+ for Si4+ in the tetrahedral sites.РезюмеБентониты мелового периода были собраны в обнажениях Свитграс Арк и Дистербд Белт в Монтане. Смешанно-слойные компоненты бентонитов иллит-смектит (И/С) из Свитграс Арк имеют 0 до 25% слоев иллита и в них не было обнаружено структурного Fе2+, тогда как пробы из Дистербд Белт имеют примерно от 25 до 90% слоев иллита, и все содержат Fе2+. Положительная корреляция (г = 0,89) существует между процентным содержанием структурного жедеза (Fе2+) и количеством фиксированного межслоя К в И/С.Большое процентное содержание слоев иллита в пробах из Дистербд Белт относится за счет реакций, присходящих при повышенных температурах, обусловленных залеганием бентонитов под надвиговыми пластами. Увеличение Fе2+/Fе3+ с повышением процентного содержания слоев иллита, на основе опыта, связывается с окислительно-восстановительной реакцией, включающей окисление органического материала. Хотя при рассмотрении всех образцов не существует статистических доказательств увеличения октаэдрического заряда с увеличением содержания слоев иллита, восстановление железа могло обусловить от 10 до 30% увеличения общего структурного заряда, который возник в каждом образце в течение метаморфизма. Остающееся увеличение структурного заряда может быть отнесено за счет замещения Аl3+ кремнием Si4+ в тетраэдрических слоях.ResümeeKreidehaltige Bentonite wurden in Zutageliegen vom “Sweetgrass Arch” und “Disturbed Belt” in Montana gesammelt. Die Illit-Smektit (I/S) Anteile der Wechselschicht der Bentonite vom “Sweetgrass Arch” haben von 0 bis 25% Illitschichten und kein feststellbares strukturelles Eisen2”1”, wohingegen die Proben vom “Disturbed Belt” ungefähr von 25 bis 90% illitschichten haben und alle enthalten Eisen2+. Eine positive Abhängigkeit (r = 0,89) besteht zwischen dem Prozentsatz des strukturellen Eisen, welches als Eisen2”1” vorkommt und der Menge der gehärteten Zwischenschicht K im I/S.Der höhere Prozentsatz von Illitschichten in den Proben vom “Disturbed Belt” wird Reaktionen zugeschrieben, die mit erhöhten Temperaturen verbunden sind, die durch Vergrabung unter Schubschichten erzeugt wurden. Die Zunahme in Eisen2+/Eisen3+ mit zunehmendem Prozentsatz von Illitschichten wird versuchsweise einer Redoxreaktion zugeschrieben, in der Oxidation von organischem Material beteiligt ist. Wenn alle Proben zusammen berücksichtigt werden, gibt es keine statistischen Beweise für die Zunahme in oktaedrischer Ladung mit Zunahme in Illitschichten. Eisenreduktion könnte jedoch soviel wie 10 bis 30% zu der Zunahme in gesamtstruktureller Ladung, welche in jeder gegebenen Probe vorkommt, beitragen. Der Rest der strukturellen Ladungszunahme kann der Substitution von Si4+ mit Al3+ in den tetraedrischen Plätzen zugeschrieben werden.RésuméDes bentonites du Crétacé ont été collectionés d’un affleurement de Sweetgras Arch et du Disturbed Belt du Montana. Les constituants illite-smectite à couches mélangées (I/S) des bentonites de Sweet-gras Arch ont de 0 à 25% de couches d’illite et pas de Fe2+ détectible, alors que les échantillons du Disturbed Belt ont de 25 à 90% de couches d’illite, et tous contiennent Fe2+. Une corrélation positive (r = 0,89) existe entre le pourcentage de fer de composition qui est Fe2+ et la quantité de K fixe intercouche dans I/S.Le pourcentage plus élevé des couches d’illite dans les échantillons du Disturbed Belt est attribué à des réactions liées à de hautes températures causées par l’enterrement sous des nappes de charriage. L’augmentation de Fe2+/Fe3+ allant de pair avec l’augmentation des pourcentages des couches d’illite est tentativement attribué à une réaction rédox impliquant l’oxidation de matière organique. Bien qu’il n’y ait aucune évidence statistique d’une augmentation de charge octaèdrique accompagnant une augmentation dans les couches d’illite lorsque tous les échantillons sont considérés ensemble, la réduction de fer peut avoir contribué de 10 à 30% à l’augmentation de la charge totale de composition qui s’est passée pendant le métamorphisme de tout échantillon. Le reste de l’augmentation de charge de composition peut être attribué à la substitution d’Al3+ à Si4+ dans les sites tétraèdriques.

Oxygen Isotope Geothermometry of the Burial Metamorphic Rocks of the Precambrian Belt Supergroup, Glacier National Park, Montana

Isotopic evidence supports the conclusions of other studies that the Precambrian Belt rocks have been subjected to high-grade diagenesis and low-grade metamorphism. Isotopic temperatures calculated from O 18 /O 16 ratios of coexisting quartz and illite range from 225°C to 310°C and are interpreted as being temperatures reached during metamorphism. Isotopic temperatures generally increase with maximum depth of burial. A plot of isotopic temperatures as a function of depth of burial can be extrapolated to 20°C (surface temperature) at approximately 5,500 m above our uppermost sample. This is consistent with the probable amount of overburden above this sample as inferred from stratigraphic evidence. The isotopic temperatures are also consistent with a model of equilibration during burial in a normal geothermal gradient. Feldspar underwent isotopic exchange during metamorphism but does not always appear to have attained isotopic equilibrium with quartz and illite. The isotopic data indicate some degree of disequilibrium between carbonate and quartz and suggest that the carbonate may have been more readily subject to retrograde exchange than was the silicate. The isotopic compositions of whole-rock samples vary with depth in the stratigraphic section, apparently reflecting post-depositional isotopic exchange.

OXYGEN ISOTOPES AND THE EXTENT OF DIAGENESIS OF CLAY MINERALS DURING SEDIMENTATION AND BURIAL IN THE SEA

Oxygen isotope ratios of <0.1-μm smectite in bottom sediments of the Mississippi River and the Gulf of Mexico near the mouth of the river have been determined to investigate diagenesis of land-derived clay minerals during sedimentation in the sea. No difference was detected in δ18O (SMOW) between the river and the Gulf samples indicating that no smectite alteration or addition of neoformed smectite to the river samples took place during sedimentation. Thus, authigenic minerals in the river sediments cannot make up more than a few tenths of a percent of the bulk sediments.Similar results were obtained from 3 × 106-yr b.p. sediments buried to 80 to 600 m at Deep Sea Drilling Project site 323, Bellingshausen Abyssal Plain. No significant change with depth was noted in the δ18O of the <0.3-μm size fraction, mostly smectite, of these land-derived sediments. On the basis of the δ18O of the deepest sample, the maximum amount of authigenic minerals in the land-derived sediments during burial in the sea cannot be more than one or two percent of the bulk sediments. Hence, the alteration at seafloor temperatures of 25-45% of the <0.1-μm size clays in 3 × 106 yr b.p. sediments reported in a previous study is not substantiated. The data demonstrate that land-derived smectite is stable in the sea, and that oxygen isotopes can be used to investigate the modes and the temperatures of formation of authigenic smectites in marine sediments that are younger than 25 × 106 yr and that formed below 25°C.

Pore-to-regional-scale Integrated Characterization Workflow for Unconventional Gas Shales

Based on recent studies of Barnett and Woodford gas shales in Texas and Oklahoma, a systematic characterization workflow has been developed that incorporates lithostratigraphy and sequence stratigraphy, geochemistry, petrophysics, geomechanics, well log, and three-dimensional (3-D) seismic analysis. The workflow encompasses a variety of analytical techniques at a variety of geologic scales. It is designed as an aid in identifying the potentially best reservoir, source, and seal facies for targeted horizontal drilling. Not all of the techniques discussed in this chapter have yet been perfected, and cautionary notes are provided where appropriate. Rock characterization includes (1) lithofacies identification from core based on fabric and mineralogic analyses (and chemical if possible); (2) scanning electron microscopy to identify nanofabric and microfabric, potential gas migration pathways, and porosity types/distribution; (3) determination of lithofacies stacking patterns; (4) geochemical analysis for source rock potential and for paleoenvironmental indicators; and (5) geomechanical properties for determining the fracture potential of lithofacies. Well-log characterization includes (1) core-to-log calibration that is particularly critical with these finely laminated rocks; (2) calibration of lithofacies and lithofacies stacking patterns to well-log motifs (referred to as gamma-ray patterns or GRPs in this chapter); (3) identification and regional to local mapping of lithofacies and GRPs from uncored vertical wells; (4) relating lithofacies to petrophysical, geochemical, and geomechanical properties and mapping these properties. Three-dimensional seismic characterization includes (1) structural and stratigraphic mapping using seismic attributes, (2) calibrating seismic characteristics to lithofacies and GRPs for seismic mapping purposes, and (3) determining and mapping petrophysical properties using seismic inversion modeling. Integrating these techniques into a 3-D geocellular model allows for documenting and understanding the fine-scale stratigraphy of shales and provides an aid to improved horizontal well placement. Although the workflow presented in this chapter was developed using only two productive gas shales, we consider it to be more generically applicable.

Mineralogy, crystallinity, O 18 /O 16 , and D/H of Georgia kaolins

Mineralogy, kaolin crystallinity, Fe content, δO18, and δD were determined for late Cretaceous “soft” and early Tertiary “hard” Georgia kaolins. The crystallinity of the <0.5-, 0.5–1.0-, and 1.0–2.0- µm size fractions of soft kaolins was higher than that of equivalent size fractions of hard kaolins. δO18 and δD of the soft and hard kaolins ranged between 18.5 to 23.1‰, and −64 to −41‰, respectively, and could not be used to discriminate soft from hard kaolins. The trends of crystallinity vs. δO18 were different for kaolins collected at different localities, and, for a given sample, δO18 generally decreased with increasing crystallinity and with increasing crystallite size. These data indicate that the Tertiary kaolins could not have been simply derived from the Cretaceous kaolins by winnowing unless post-sedimentation recrystallization of one or both occurred. δD vs. δO18 systematics indicate that the late Cretaceous to early Tertiary Georgia kaolins crystallized over a temperature range of about 15°C in the presence of waters that varied little in isotopic composition.РезюмеБыли определены минералогия, степень кристаллизации каолина, содержание Fе, δО18, и δD для “мягкого” позднемелового и “твердого” раннетретичного джорджийских каолинов. Кристальность фракций мягких каолинов размером <0,5-, 0,5-1,0-, и 1,0–2,0-рт была выше, чем кристальной» эквивалентных по размеру фракций твердых каолинов. δО18 и δD мягких и твердых каолинов колебались от 18,5 до 23,1% и от 64% до 41% соответственно и не могли быть использованы для распознавания мягких каолинов от твердых. Характер зависимости кристальности от δО18 был разный для каолинов, отобранных из разных мест, и для данного образца δО18 в основном уменьшается при увеличении кристальности и при увеличении размера кристаллитов. Эти данные указывают на то, что третичные каолины не могли просто формироваться из меловых каолинов путем механического фракционирования пока не произошла послеседиментационная перекристаллизация одного типа или обоих. δD в зависимости от δО18 показывают, что позднемеловые и раннетретичные каолины кристаллизировались в диапазоне изменений температуры около 15°С в присутствии вод, незначительно отличающихся по составу изотопов. [Е.G.]ResümeeEs wurde die Mineralogie, die Kaolinkristallinität, der Fe-Gehalt, die δO18- und δD-Werte an “weichen” Georgia-Kaolinen aus der späten Kreide und an “harten” Georgia-Kaolinen aus dem frühen Tertiär untersucht. Die Kristallinität der weichen Kaoline der Fraktionen <0,5; 0,5–1,0, und 1,0–2,0 µm war besser als die der entsprechenden Kornfraktionen der harten Kaoline. δO18 und δD der weichen und harten Kaoline lag zwischen 18,5 und 23,1‰ bzw. zwischen −64 bis −41‰ und konnte nicht zur Unterscheidung zwischen weichem und hartem Kaolin verwendet werden. Wurde die Kristallinität gegen δO18 aufgetragen, so waren die Trands für Kaoline von verschiedenen Vorkommen verschieden, und—bei einer gegebenen Probe—nahm der δO18-Wert im allgemeinen mit zunehmender Kristallinität und mit zunehmender Kristallgröße ab. Diese Daten deuten darauf hin, daß die tertiären Kaoline nicht einfach durch Sortierung aus den Kaolinen der Kreide entstanden sein können, ohne daß eine postsedimentäre Rekristallisation des einen oder beider Kaoline eintrat. Darstellungen von δD gegen δO18 zeigen, daß die spätkretazischen bis frühtertiären Georgia-Kaoline über einen Temperaturbereich von etwa 15üC in Gegenwart von Wässern kristallisierten, die in ihrer Isotopenzusammensetzung in geringem Maße variierten. [U.W.]RésuméOn a déterminé la minéralogie, la cristallinité de Kaolin, le contenu en Fe, δO18, et δD pour des kaolins de Georgie “mous” du bas Crétacé et “durs” du haut Tertiaire. La cristallinité de fractions de taille <0,5, 0,5–1,0 et 1,0–2,0 µm de kaolins mous était plus élevée que celle de fractions de tailles equivalentes de kaolins durs. δO18 et δD des kaolins mous et durs s’étendaient entre 18,5 à 23,1‰, et −64 à −41‰ respectivement, et ne pouvaient pas être employés pour discriminer entre les kaolins mous et les kaolins durs. Les tendances de cristallinité vs. δO18 étaient différentes pour les kaolins rassemblés à des localités différentes, et, pour un échantillon donné, δ18 diminuait généralement proportionnellement à une augmentation de cristallinité et à une augmentation de la taille de la cristallinité. Ces données indiquent que les kaolins Tertiaires ne peuvent pas être simplement dérivés des kaolins Crétacés, par ruissellement à moins que la recristallisation de l’un ou l’autre ne se soit produite. Les systématiques de δD vs. δO18 indiquent que les kaolins de Géorgie du bas Crétacé au haut Tertiaire se sont cristallisés sur une étendue de températures d’à peu près 15°C en la présence d’eaux qui ont varié peu de composition isotopique. [D.J.]

Evidence for the Formation of Illite from Smectite During Burial Metamorphism in the Belt Supergroup, Clark Fork, Idaho

ABSTRACT Mineralogic and chemical data have been obtained for 26 argillite samples collected from an 8500 meter section of the Precambrian Belt Supergroup, Clark Fork, Idaho. Minerals present are quartz, feldspar, illite, and calcite ± calcite and/or dolomite. Illite/quartz and chlorite/quartz ratios decrease up-section supporting the conclusion of Boyce (1973) that the Belt section represents an ancient prograding delta. The ratio of K-feldspar to total feldspar decreases from 20-80 percent in the upper two-thirds of the section to 0-10 percent in the lower one-third. The mechanism proposed for the decrease in K-spar down-section is the conversion of K-poor smectite into K-rich 1Md illite. A later higher-temperature reaction converted much of the 1Md illite into 2M illite (muscovite). e suggest that a low K-spar/plagioclase ratio in an illite-containing shale suite indicates that the illite formed from smectite during metamorphism rather than from the direct weathering of some high-temperature phase such as feldspar. Most continental volcanic material is produced during orogenic episodes. Most smectite forms from volcanic material and smectites will be converted into illites if enough time has elapsed for them to have been involved in one or more geosynclinal cycles. Therefore, the proportion of smectite in sediments through time can be related to the timing of orogenic episodes. This model works fairly well in North America where smectite maxima coincide with orogenic episodes, becoming smaller with increasing antiquity of the orogeny.

An X-ray Technique for Distinguishing Between Detrital and Secondary Quartz in the Fine-grained Fraction of Sedimentary Rocks

ABSTRACT Fine-grained disseminated quartz in sedimentary rocks may often be characterized as being secondary in origin on the basis of its X-ray diffractogram. The existence of a diffractogram intensity ratio I100/I101 substantially greater than that obtained from a random sample of crushed quartz appears to be a sufficient (but not necessary) criterion for recognizing quartz of secondary origin in the absence of a volcanic component. The intensity ratio anomaly reflects preferred orientation of quartz in the sample mount and results from the presence of faces on the (100) form (prism face) on some of the grains. These results are consistent with both geologic and oxygen isotopic evidence.

Oxygen and hydrogen isotope geochemistry of Cretaceous bentonites and shales from the Disturbed Belt, Montana

The mineralogy, δO18, and δD of the <0.1 μm fraction of 22 Cretaceous bentonites and the mineralogy and δO18 of the < 0.1 μm fraction of 14 adjacent shales collected from outcrops in the Sweetgrass Arch and Disturbed Belt, Montana, have been determined. Mixed-layer illite/smectite (I/S) is the dominant mineral in the bulk bentonite and usually the only mineral in the < 0.1 μm fraction. I/S is also the major clay mineral in the shales. The diagenetic grade in bentonite is qualitatively given by the percentage of illite layers in I/S, which varies from 2 to 25 (Sweetgrass Arch) to as high as 95 (Disturbed Belt). δO18 of < 0.1 μm bentonite generally decreases from about +20%. to about +13%. with increasing diagenetic grade. On a plot of δD versus δO18, data for the < 0.1 μm bentonite define a field that generally parallels, but falls on the meteoric water line side of the smectite-water line (Savin and Epstein, 1970). δO18 of bulk bentonite is 1 to 3%. more negative than the δO18 of the < 0.1 μm fraction, due to the presence of volcanic quartz and feldspar. δO18 of several size fractions of clay-sized quartz separated from the bentonite varies from +11%. to +24%., and, in a given bentonite, generally increases with decreasing grain size. Among the different bentonites, the δO18 range of the different grain sizes decreases as the percentage of illite layers in the coexisting I/S increases. The δO18 of 0.1–0.5 μm shale quartz is generally 1 to 4%. more positive than clay-sized quartz from an adjacent bentonite, and the δO18 of < 0.1 μm I/S concentrate of shales is generally < 1 to 4%. more negative than the < 0.1 μm I/S from an adjacent bentonite. Isotopic temperatures, interpreted to be maximum burial temperatures, range between about 160°C (shale), to about 250°C (bentonite). The isotopic data can be interpreted using the stages: 1. 1) deposition of volcanic glassy ash containing some quartz and feldspar; 2. 2) devitrification into mostly 100% expandable smectite with a δO18 of about 20%. and δD of −60 to −80%.; 3. 3) tectonic burial beneath thrust sheets in the Disturbed Belt region which induced elevated temperatures that allowed oxygen and hydrogen isotope reequilibration as the percentage of illite layers in I/S increased and diagenetic quartz formed; and 4. 4) exhumation of the strata with preservation of O18O16 and D/H attained during burial.